Problem: Simplify the following expression: $t = \dfrac{6y}{7} - \dfrac{4y}{9}$
Explanation: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $7$ and $9$ $\lcm(7, 9) = 63$ $ t = \dfrac{9}{9} \cdot \dfrac{6y}{7} - \dfrac{7}{7} \cdot \dfrac{4y}{9} $ $t = \dfrac{54y}{63} - \dfrac{28y}{63}$ $t = \dfrac{54y -28y}{63}$ $t = \dfrac{26y}{63}$